Field of the Invention
The present invention relates to a process for the preparation of citral. More particularly, it relates to a process for the preparation of citral by cracking of prenal diprenyl acetal in a vapor phase.
The citral thus obtained is useful as an intermediate in the synthesis of vitamin A. It can also be used for its aromatic properties.
The preparation of citral by cracking of prenal diprenyl acetal in a liquid phase is disclosed in the prior art. For example, the preparation of aldehydes containing .alpha.,.beta.-ethylene bonds by reacting an aldehyde containing an .alpha.,.beta.-ethylene bond and an allyl alcohol at a high temperature in the liquid phase is disclosed in French Patent No. 2,160,125. The aldehyde containing an .alpha.,.beta.-ethylene bond may be 3,3-dimethylacrolein, which is also known by the name prenal, and the allyl alcohol may be 3-methyl 2-buten-1-ol known by the name prenol.
The condensation reaction is preferably carried out in the presence of an acid catalyst at a temperature between 100.degree. and 250.degree. C. The acid used as a catalyst is preferably selected from inorganic acids such as sulphuric, phosphoric, halogenated, nitric, sulphurous, phosphorous, perchloric, boric, silicic, and salts of acids containing a dissociable hydrogen. Organic acids can also be used. The quantity of acid used in the case of an inorganic acid with a pK of 0 to about 3 is preferably between 0.01 and 0.5%. Other patents, such as French Patent No. 1,582,515 describe variants and improvements to this type of process.
The preparation of citral by heating diprenyl acetal in the presence of phosphoric acid and an inert liquid which has, at the pressure of the reaction, a boiling point greater than that of prenol and lower than that of citral, is disclosed in U.S. Pat. No. 4,288,636. The inert liquid is preferably 3,3,7-trimethyl-4-oxa-1,6-octadiene. The quantity of phosphoric acid used is between 0.001 and 0.5 % by weight, and preferably between 0.005 and 0.05 % by weight.
However, these liquid phase processes for preparing citral have many disadvantages. In particular, because of the large volume of liquid which must be used, the difficulty or even the impossibility of carrying out the reactions continuously, and also the high cost of the operations, these processes are difficult to exploit industrially.
To avoid these disadvantages, the present invention provides a process for the preparation of citral by cracking prenal diprenyl acetal in a vapor phase, in the presence of a heterogeneous type of acid catalyst.
An important aspect of the present invention resides in the choice of the catalyst, which is important to the efficacy of the reaction. We have observed that in the presence of catalysts exhibiting an acidity which is too high, the citral was converted to secondary cyclic products such as para-cymene. This reaction of degradation of citral has been previously observed by Clark et al.; Tetrahedron, Vol. 33 p. 2187, (1977); who identified 14 cyclic products obtained from citral in acid media. More particularly, the authors studied the degradation products of citral in the presence of hydrochloric acid, at pH values between 2.4 and 3.2. They showed the presence of novel products, and proposed a reaction pathway. However, the list of products obtained is not exhaustive, and that products not yet identified must be formed.